Take a Journey of Discovery with RXTE - Classroom Activity

Take out the list of things you observed during your visit to the black hole.

Here are explanations for some of the things you may have observed.

The Drone

Light

The drone moved slower and slower as it approached the black event horizon. We never see it actually reach the event horizon. The light we do receive
from our drone is shifted in energy by the extreme gravity. We observed
our drone's green light become yellow, then red, and then move beyond the
visible range into the infrared, and then radio range of light. The energy
of the light our drone is emitting is lowered because as the photons of light
travel from the drone to where we observed them, they
have to work harder against the black hole's gravitational pull, in order
to still travel at the speed of light. However, from the drone's perspective,
which we saw on the video screen, the light's energy was not shifted at all.
This is why we eventually lost sight of the drone - the light it was giving
off and reflecting lost enough energy that it moved out of the range of
light that our eyes can see.

Time

From our spacecraft, we also observe the drone moving slower and
slower as it nears the event horizon. A clock on the drone would
appear to run slower as well, though from the drone's perspective, it
would appear normal. Upon reaching the event horizon, from our
perspective, the clock would appear to stop entirely. The drone would
be frozen in time - and we would never see it enter the event horizon.
Still, from the drone's frame of reference, time appears to run at a
normal rate. This effect is called time dilation.

The Companion Star

We also observed that the black hole has such strong gravity that it
has pulled matter of its red companion star, essentially cannibalizing
it. The matter swirls around the black hole, forming an accretion
disk, and is heated up to millions of degrees, emitting
X-rays, before disappearing beyond the event horizon forever.

The Singularity Itself

The black hole itself, the singularity or point with infinite
density and gravity, that is the collapsed core of the star, is
invisible. Because the gravitational field of the black hole is so
strong, the escape velocity, the speed with which something
would have to travel to leave the surface of a body, exceeds the speed
of light. This means that even light cannot travel fast enough to
escape the black hole's grip.

But, the grip of black hole does not extend as far as one would think.
A black hole is not a cosmic vacuum cleaner. If fact, if our Sun were
replaced with a black hole, it would not have an effect on us,
gravitationally speaking. This is because the surface of the Sun lies
beyond its Schwarzchild radius. This is the radius at which the
escape velocity equals the speed of light, and within which the object
can no longer be seen. Every object has a Schwarzchild radius - it is
simply the radius to which the object would have to be compressed for
it to become a black hole. It is proportional to an object's mass.
The Schwarzchild radius of the Earth is 1 cm. If the Earth was
crushed down to a radius of 1 cm, its escape velocity would be the
speed of light.

The surface of an imaginary sphere with radius equal to the Schwarzchild
radius and centered on a collapsing star is called the event horizon.
It defines the region withing which no event can ever be seen, heard, or
known by anyone outside it.

But What is Inside?

Unfortunately, there is no way to know what is inside the event horizon of a
black hole - as observers, the time dilation effect, as well as the
gravitational shifting of light, keep us from being able to even watch
an object break through the event horizon. If we sent a volunteer astronaut
down, it would a one way trip, with no means for him or her to report back.
What would they see as they were falling in? Surprisingly, nothing
particularly interesting. Images of faraway objects may be distorted in
strange ways, since the black hole's gravity bends light, but that's about it.
In particular, nothing special happens at the moment when you cross the
horizon. Even after you've crossed the horizon, you can still see things
on the outside: after all, the light from the things on the outside can
still reach you. No one on the outside can see you, of course, since the
light from you can't escape past the horizon.

Once our astronaut was through the event horizon, they would have to hit
the singularity eventually, and before they get there, there will be
enormous tidal forces - forces due to the curvature of spacetime - which
will squash our astronaut in some directions and stretch them in others
until they look like a piece of spaghetti. At the singularity all of
present physics is mute as to what will happen, but our astronaut won't care.
They'll be dead.

Want to Know More?

This list of frequently asked questions (FAQ) has a great
description of what it would be like to jump into a black hole - or
just watch someone else - http://cfpa.berkeley.edu/BHfaq.html